This invention relates to multiwavelength devices and more particularly to devices that are capable of detecting or generating more than one optical wavelength.
In order to expand the transmission capabilities of optical fiber systems, it will be desirable to transmit and receive more than one wavelength on a single optical fiber. To accomplish this, several prior art proposals have used classical optical components such as lenses, prisms, gratings, or interference filters. These prior art systems all suffer from the necessity of using multiple components, each one of which must be capable of passing the wavelength of interest and coupling this wavelength to an ultimate photodetector or optical source. It would be highly desirable if the detection and demultiplexing or generation and multiplexing of multiple wavelengths could take place in a single integrated device.
The problem of detecting several wavelengths in a single incoming wave has been solved using several photodetectors in the solar energy art, see for example, U.S. Pat. No. 2,949,498 to E. D. Jackson, issued Aug. 16, 1960. This apparatus by Jackson uses several semiconductor elements that are connected in series from an electrical standpoint, and the incoming radiation is coupled to the first element in a tandem arrangement of elements with the element having the largest bandgap being placed first in the series of elements. As a result, the first element detects the optical energy from the incoming radiation with photons having energy equal to or greater than the bandgap of the material in this first element. The following elements in the series extract other energy from the incoming optical wave whose photons have energy equal to or greater than their respective bandgaps.
This approach of using individual semiconductor devices has been followed in the copending application entitled "Optical Monitoring Photodetector System", by R. C. Miller and B. Schwartz, filed June 19, 1978, Ser. No. 916,501 which application is assigned to the same assignee as the present application. In this Miller et al application two photodetectors are coupled together by an electrically insulating layer 154 in FIG. 3, and the active layers of each photodetector are arranged such that the first active layer to be encountered by radiation incoming and reflected from an optical fiber 108 has the larger bandgap. In the Miller et al application the layer closest to the optical fiber is used in a feedback circuit to control the laser source in response to light energy reflected from the fiber, and the layer furthest from the optical fiber is used to detect information that is transmitted from a remote source over the optical fiber. This device, however, like the apparatus in the Jackson patent is really two separate photodetectors 150 and 152 that have been sandwiched together for the purpose of providing optical alignment. It would be highly desirable to have a single integrated structure that would provide a device capable of demultiplexing and detecting separate wavelengths present in an optical fiber.